This invention relates to the process of shimming, and more particularly, to a process of determining the shape and thickness of a shim.
Shims, also called fillers, are used to fill voids discovered during an assembly process. Voids are formed by the misalignment of parts during assembly or by the incorrect manufacture of the parts being assembled. Although mostly used on an informal basis during manufacturing, some shims are called out on drawings as part of the manufacturing process. Filling voids between mating surfaces on assembled parts results in a more structurally sound assembly.
Shims are used throughout the aerospace industry to compensate for part variation due to the complex aerodynamic shapes of various assembled parts. An example of the shimming within the aerospace industry is at the point where the skin of the airplane strut is attached to a torque box. Currently, the skin is moved into place, and shim thickness is measured using a feeler gauge. Use of the feeler gauge requires that the skin and the strut torque box be assembled and that the gauge be inserted between a pair of mating surfaces on the strut torque box and skin. When a feeler gauge is used, a range of gauges, and multiple measurement attempts, are usually required until the correct thickness is determined because of tight space constraints within the assembled parts. Once three or four locations associated with a shim are measured, the shim is constructed by hand. There are approximately 116 shims required in each strut torque box to torque box skin assembly. Because each shim must be measured for and constructed individually, it typically takes 100 to 200 man-hours to shim each strut torque box and skin assembly.
Consequently, there is a need for a method and apparatus capable of determining the shape and thickness of a shim with efficiency and accuracy.
The present invention discloses a method and apparatus for determining the shape and thickness of a shim to be fit between two surfaces, a strut torque box and torque box skin for example. The method and apparatus allow the simultaneous measurement of a plurality of retro-reflective targets on the strut torque box using digital photogrammetry equipment. The targets are located on the shim surfaces of the strut torque box and the measurements yield a plurality of measured points. The locations of these measured points are transformed to yield the shape of the shim required at each shim location without assembly of the strut torque box and torque box skin.
In one embodiment, the present invention includes an apparatus for determining the shape of the shim that can be inserted between a first body and a second body in an interference fit. The apparatus includes a processing element for receiving a plurality of measured points that define a surface of the first body. The processing element also uses an engineering surface and creates a vector passing through respective points of a group of the measured points and normal to the engineering surface. The processing element determines, for each point of the group, a distance between the engineering surface and a point that is local to the respective point in the group that is most outboard from the engineering surface. The processing element also constructs a plurality of new points with each new point associated with a respective point of the group and displaced the respective distance along the respective vector. The processing element further constructs a second surface of the second body from the plurality of new points so that the shape of the shim is defined between the plurality of the measured points and the first surface.
In another embodiment, the present invention includes an apparatus for determining the shape of a shim that can be inserted between a first body and a second body. The apparatus comprises a processing element for receiving a plurality of the measured points that define a first surface of the first body. The processing element also uses an outboard mating surface of the second body and creates a vector passing through each point of a group of the measured points and normal to the outboard surface. The processing element determines, for each point of the group, a distance between the outboard mating surface and a point that is local to the respective point in the group and is most outboard from the outboard surface. The processing element also constructs a plurality of new points with each new point associated with a respective point of the group and displaced the respective distance along the respective vector. The processing element realigns the measured points until each point of the group of the measured points is coincident with each new point on the same vector so that the shape of the shim is defined between the plurality of measured points and the outboard mating surface.
In another embodiment, the apparatus includes a plurality of targets each marking a position for the shim on the first body and a measurement device positioned to measure the plurality of targets and construct the plurality of points from the measured targets. The measurement device can be a photogrammetry device measuring the plurality of targets by capturing an optical image. Preferably, the plurality of targets are retro-reflective targets having a contrast detectable by the photogrammetry device. The photogrammetry device may include one, or a plurality, of cameras.
In yet another embodiment, the processing element repositions the new points until the measured points protrude less than an engineering tolerance into either the second surface or the outboard mating surface. Repositioning configures the shim to form an interference fit between the first and second bodies.
In another embodiment, the second body includes a tool-locating detail and the processing element constructs the outboard mating surface from a position of the tool locating detail and a known displacement of the outboard mating surface with respect to the tool locating detail. In one aspect, the tool locating detail is located on the hinge of a thrust reverser of a jet aircraft engine.
In still another embodiment, the present invention includes an apparatus for determining the shape of a shim that can be inserted between a first body and a second body. The apparatus comprises a plurality of targets, a measuring device and a processing element. The plurality of targets are positioned at a plurality of shim points on the first body. The measuring device is positioned to measure the plurality of targets and created a plurality of measured points corresponding to the plurality of targets. The plurality of points define a first surface on the first body. The processing element receives the plurality of measured points. The processing element uses a second surface and transforms the locations of the measured points with respect to the second surface so that the thickness of the shim is defined between the plurality of points and the second surface. In one aspect, the transformation performed by the processing element is a maximum material best fit with constraints on the plurality of measured points to one side of the second surface of the second body.
The present invention has the advantage of allowing the calculation of multiple shim shapes with a single set of measurements. Also, the present invention allows for the shim shape to be determined without assembly of the component parts. These efficiencies result in a large decrease in the time and effort required for shim installation, particularly in aerospace applications which require the assembly of large and complex shapes.